Time variable alert tone circuit for personal radio paging systems

ABSTRACT

The present invention concerns a method for varying over a period of time the alert tone in a personal radio-paging receiver making extensive use of signal voltages already present in the circuit and decreasing the overall drain on the receiver supply battery.

United States Patent Steven M. Baer Red Bank, NJ.

Sept. 30, 1968 Apr. 6, I97 1 Bell Telephone Laboratories, Incorporated Murray Hill, Berkeley Heights, NJ.

Inventor Appl. No. Filed Patented Assignee TIME VARIABLE ALERT TONE CIRCUIT FOR PERSONAL RADIO PAGING SYSTEMS 3 Claims, 2 Drawing Figs.

US. Cl 325/392, 331/106, 340/311, 340/384 Int. Cl G08b 3/ 10 Field of Search 325/55, 64,

364, 392,466, 51 m), 54 (iii); 340/171, 31 384 (E); 331/106, 47,55, 145, 149, 153, 182

Primary Examiner-Robert L. Griffin Assistant Examiner-Benedict V. Safourek Attorneys-R. J. Guenther and E. W. Adams, Jr.

ABSTRACT: The present invention concerns a method for varying over a period of time the alert tone in a personal radiopaging receiver making extensive use of signal voltages already present in the circuit and decreasing the overall drain on the receiver supply battery.

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V0 LTS SOUNDER SOUNDER TIME OUTPUT OUTPUT BEGINS ENDS TIME VARIABLE ALERT TONE CIRCUIT FOR PERSONAL RADIO PAGING SYSTEMS BACKGROUND OF THE INVENTION This invention relates to personal radio-paging receivers and, more particularly, to time variable alert tone circuits for use in such receivers.

As commonly employed, such personal radio-paging receivers emit a fixed volume audible tone when the user is paged. The basic design features of such receivers are discussed in an article by A. E. Kerwein and L. H. Steiff appearing at 42, Bell System Technical Journal 527 (May 1963).

LII

In the usual configuration the sounder operates at a fixed unit accordingly. Therefore, it is desirable to provide a meansof producing a time variable alert tone which would begin at a volume level low enough not to cause annoyance in quiet environments and increase during the time period, unless terminated by the user, to a level high enough to be audible even in very noisy locations.

Also in portable-receiving systems, it is desirable to increase the operating life of the supply battery which serves as the sole source of operating energy.

SUMMARY OF THE INVENTION The present invention involves a relatively simple means for producing a time variable alert tone output in a personal radio-paging receiver. The invention utilizes voltage forms produced by the existing timer circuitry for bias and signal sources, and the added circuitry in operation actually reduces the average power drain on the battery. In accordance with a particular embodiment of the invention the sole additional circuitry comprises an inverter stage containing one transistor, two resistors, and a diode. This circuit obtains a monotonically decreasing voltage from the existing timing multivibrator circuit and converts it into a monotonically increasing bias source for the alert tone power amplification stage. This variable bias modifies the gain of the power amplifier stage, thus producing a sounder output proportional to bias and hence time.

A further feature of the invention resides in the fact that the voltage supply for the inverter stage transistor is derived from the substantially constant amplitude DC signal also produced by the timer circuitry. This signal is normally used solely to DESCRIPTION OF THE FIGS.

FIG. 1 is a schematic drawing of a personal radio-paging receiver containing the present invention;

FIG. 2 is a graphical representation of certain voltages in the circuit as a function of time.

DETAILED DESCRIPTION Referring to FIG. 1, the alert signal from the central transmitter is received by antenna 1 and the radio frequency receiver 2. This signal is then applied to the selector and detector 3. If it is of the proper frequency, it causes the detector to emit an AC signal, the negative portion of which is applied to trigger the timing multivibrator 6.

Transistors 10 and 20 form the heart of the monostable timing multivibrator. The particular multivibrator chosen is of the complementary symmetry type, with transistor 10 being NPN and transistor 20, PNP. The relevant feature of this variety of multivibrator is that in the absence of any external signal, both transistors are nonconducting. Thus, battery drain is minimized.

When the multivibrator is in the OFF state, the voltage at the emitter and base of transistor 10, at the collector of transistor 20, and at all three electrodes of transistor 30 is zero. The emitter and the base of transistor 20, together with the collector of transistor 10, are at the supply potential, +2.5 volts.

The alert tone triggeringsignal, a negative voltage pulse, is applied through the input coupling circuit composed of resistor 4 and diode 5 to the base of transistor 20. Diode 5 is used to block the positive portion of the AC triggering signal from the base of transistor 20; without it that transistor would be turned OFF prematurely by the positive half of the triggering signal. The action of the negative signal forward biases the base-emitter junction of transistor 20.and puts that transistor into saturation. When transistor 20 goes into saturation, its emitter-collector resistance drops to near zero and therefore its collector voltage rises to nearly +2.5 volts, the voltage at the emitter of transistor 20. The very steep pulse which appears at the collector of transistor 20 is passed by capacitor 15 onto the base of transistor 10, forward biasing that transistor and causing it to go into saturation. When transistor 10 goes into saturation, its collector voltage drops from +2.5 volts to near zero. This action regenerates the original stimulus and thereby holds transistor 20 in a forward biased condition even though the triggering pulse passes. So long as transistor 20 is in saturation the collector voltage remains constant at nearly +2.5 volts and the voltage at the right face of capacitor 15 is likewise held at nearly +2.5. Since the left face of capacitor 15 is connected through resistor 12 to ground, the positive voltage on the left face of capacitor 15 caused by the initial pulse gradually decreases. This produces a voltage pattern at the base of transistor 10 in the shape of a decaying exponential curve, with a starting value of nearly +2.5 volts appearing at the instant that transistor 20 goes transistor and a time constant determined by the values chosen for capacitor 15 and resistor 12. This is illustrated by the line V in FIG. 2, showing the voltage over time at point B. When the base voltage of transistor I0 dips below the cutoff value (approximately +0.5 volts) that transistor ceases conducting. When this occurs, the collector voltage at transistor 10, no longer tied by a low resistance path to the emitter voltage, rises to nearly +2.5 volts, removing the net forward bias on transistor 20, thereby causing it to turn OFF, returning the multivibrator to standby. The time constant is normally chosen to give approximately l5 seconds of ON time.

During the ON period, before the base voltage of transistor 10 drops below the cutoff value, the voltage at the emitter of transistor 10, measured across the emitter resistor 14, is a decaying exponential similar to and displaced downward from the voltage pattern at the base of that transistor. This is illustrated by the curve V in FIG. 2 showing the voltage over time measured at point C. Applicant, in accordance with the present invention, applies this voltage to the base of the inverter stage transistor 30, an NPN type. The collector supply voltage of transistor 30 is the constant amplitude DC voltage which appears during the ON period at the collector of transistor 20. This voltage is also used as the source supply for the tone oscillator 19. It is illustrated in FIG. 2 as the curve V of voltage over time measured at point A. The emitter of transistor 30 is connected to ground through resistor 18. Because the collector voltage source chosen is at zero except during the timing period, transistor 30 operates only while the multivibrator is in its unstable ON configuration. During this time the collector voltage at transistor 30 is a rising exponential ramp. This is shown in FIG. 2 by the line V the voltage curve at point D. This ramp, after passing through a voltage divider composed of resistors 22 and 23, is used as the bias voltage source for amplifier 24. The signal source for the amplifier is the output signal from the oscillator 19, coupled across capacitor 21. The gain of amplifier 24' is gradually increased by the rising ramp, causing the signal voltage through the sounder 25, and hence its output, to increase over time during the ON period of the multivibrator. When transistor 20 turns OFF, a negative pulse appears at its collector which is coupled through capacitor 15 to the base of transistor 10. Diode 13 is present to protect the base of transistor from being driven to a potentially destructive negative voltage by that pulse.

Premature termination of the sounder signal is made possible by pushbutton '11. When activated by the paged user, pushbutton l1 grounds the left face of capacitor 15, thereby causing the voltage at the base of transistor 10 to drop to zero. This turns OFF that transistor and the entire described system. Resistors 17 and 18 and diode l6 regulate the operation of the inverter stage transistor 30. Raising the value of resistor 18 or lowering the value of resistor 17 raises the initial voltage at the collector of transistor 30 and hence the initial sounder volume level. Listener tests indicate that the most satisfying increase in sounder levels is produced by a slow voltage increase at the collector of transistor 30 from H .5 to +2.0 volts over the second tone period. Just before the end of the timing interval, the base input to transistor 30 is no longer sufficient to keep this transistor ON, and without the presence of I diode 16 the collector voltage of transistor would rise to the supply voltage, nearly +2.5 volts. Diode 16 provides a constant forward voltage drop of approximately +0.5 volts over the whole tone period, adjusting the collector output voltage to the most effective variable gain region of amplifier 24. This provides a more gradual increase in sounder amplitude than would otherwise occur.

Because the present invention allows the sounder to be operated over much of the timing interval at a reduced volume level, the average volume level and therefore the power requirements, are reduced. This is done without adding significant new components, and the net effect is a reduced drain on the supply battery.

Although the invention has been described with reference to a specific embodiment thereof, it is to be understood that numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

I claim:

1. A time variable alert tone circuit for a personal radiopaging receiver comprising a circuit having an OFF state and capable of being triggered into an unstable ON state, means responsive to a signal indicating that said receiver is being paged for triggering said circuit into said ON state, means for deriving from said ON state a first DC signal for a given time period, means for deriving from said ON state a second DC signal having an amplitude that varies monotonically during said period, means responsive to said first signal for producing an audible indication that said receiver is being paged, and means responsive to said second signal for increasing the amplitude of said audible indication during said period.

2. A circuit as defined in claim 1 wherein the means responsive to said signal indicating that said receiver is being paged for producing said first and second signals comprises a monostable multivibrator, said first signal being derived from a point in said multivibrator where the output voltage is a DC pulse of substantially constant amplitude over said time period, and said second signal being derived from a point in said multivibrator where the output voltage varies monotonically during said period.

3. A circuit as defined in claim 2 wherein said amplitude of said second signal decreases monotonically during said period in combination with a voltage inverter stage, said inverter stage deriving its supply voltage from said first DC signal. 

1. A time variable alert tone circuit for a personal radiopaging receiver comprising a circuit having an OFF state and capable of being triggered into an unstable ON state, means responsive to a signal indicating that said receiver is being paged for triggering said circuit into said ON state, means for deriving from said ON state a first DC signal for a given time period, means for deriving from said ON state a second DC signal having an amplitude that varies monotonically during said period, means responsive to said first signal for producing an audible indication that said receiver is being paged, and means responsive to said second signal for increasing the amplitude of said audible indication during said period.
 2. A circuit as defined in claim 1 wherein the means responsive to said signal indicating that said receiver is being paged for producing said first and second signals comprises a monostable multivibrator, said first signal being derived from a point in said multivibrator where the output voltage is a DC pulse of substantially constant amplitude over said time period, and said second signal being derived from a point in said multivibrator where the output voltage varies monotonically during said period.
 3. A circuit as defined in claim 2 wherein said amplitude of said second signal decreases monotonically during said period in combination with a voltage inverter stage, said inverter stage deriving its supply voltage from said first DC signal. 